The present invention relates to improved curable organosilicone resin coating compositions having low solvent content. More particularly, the present invention relates to compositions comprising a conventional hydroxyl functional organosiloxane resin copolymer, wherein part or all of the solvent ordinarily present in coating compositions of the resin copolymer is replaced with a reactive liquid silicone resin having an increased flash point.
Silicone resins having trifunctional siloxy units and difunctional siloxy units are well known in the art and have been used extensively in the formulation of paints, varnishes, molding compounds and encapsulating compositions. Typically, these resins contain residual hydroxyl functionality, and occasionally residual alkoxy functionality, which may be condensed by means of heat and/or catalysis in order to cure the resins. Resins of this type are generally utilized in the form of solutions in organic solvents since they are often solids, or, at best, very viscous liquids at ordinary temperatures.
Thus, for example, U.S. Pat. No. 3,846,358 to Roedel discloses a process for producing a solid silicone resin by a complex sequence comprising partially hydrolyzing and alkoxylating an organohalosiloxane mixture with an alcohol and water, adding more alcohol, removing the acid formed by distillation adjusting the acidity of the intermediate, adding additional water and alcohol and finally adjusting the solids content.
In U.S. Pat. No. 4,160,858, Roedel discloses a similar process for producing a solventless silicone resin having a viscosity of 200 to 5,000 cP at 25.degree. C. wherein an alkali metal hydroxide is emPloyed to reduce the acidity and excess alcohol and water are stripped off in a final step.
Silicone resins may also be prepared directly from alkoxysilanes and polysiloxanes, as shown in U.S. Pat. No. 4,113,665 to Law et al. which teaches binder compositions for chemically resistant coatings. These binder resins are formed by reacting a trialkoxysilane with an aliphatic polyol and/or a silicone intermediate having hydroxyl or alkoxy terminal groups and a molecular weight of about 500 to 2,000 in the presence of an aqueous acidic solution.
Solventless silicone coating compositions are disclosed by Saad et al. in U.S. Pat. No. 4,780,338. In this invention, a silicone resin containing hydroxy, alkoxy or acyloxy functionality is blended with a diorganopolysiloxane fluid having functional groups which react with the alkoxy or acyloxy groups of the silicone resin and a condensation catalyst to cure the composition.
GB No. 2,067,212 A to Toray Silicone Company discloses room temperature curing silicone resins comprising hydroxy-functional organopolysiloxane resin, low molecular weight hydroxyl-terminated diorganopolysiloxane and an organosilane having 2 or 3 hydrolyzable groups, or a partial hydrolysis product thereof. These compositions can be cured using tin or titanate catalysts.
The use of various acids and bases as catalysts in the polymerization of silanol-terminated siloxanes is known in the art. In this regard, perfluoroalkyl sulfonic acids, inter alia, have been utilized to promote the condensation of silanol-functional siloxanes to form high molecular weight fluids, gums and copolymers. In U.S. Pat. No. 4,508,887, Kohl discloses a method for preparing a polyorganosiloxane by reacting an inert medium mixture of at least one hydroxyl-containing organosiloxane in the presence of a catalytically effective amount of a catalyst consisting essentially of an amine salt of an acid and an unreacted acid selected from fluorinated alkanesulfonic acids or sulfuric acid.
Such acids have also found utility as catalysts during the partial hydrolysis of chlorosilane mixtures. German Laid Open Publication DT 2,345,923 Al to Bayer AG discloses chlorine-functional fluids and resins prepared by the partial hydrolysis of various chlorosilanes in the presence of perfluoroalkyl sulfonic acids.
U.S. Pat. No. 4,585,705 to Broderick et al. discloses a release coating composition comprising a conventional hydroxyl functional siloxane resin containing methyl, phenyl, phenylmethyl and diphenyl siloxane units, the reactive diluent methyltrimethoxysilane, a fluid polysiloxane "slip" component and a curing catalyst. The conventional hydroxyl functional siloxane resins are generally solids and must be dissolved in an inert solvent in order to be effectively aPplied as a coating composition. Thus, the above cited patent to Broderick et al. advanced the art in that a reactive solvent replaced some, or all, of the usual inert solvent. In this way, a 100% reactive coating composition could be obtained. However, an inherent limitation of such a composition remains: the volatility of the methyltrimethoxysilane is actually greater than some of the conventional solvents it replaces (methyltrimethoxysilane has a closed cup flash point of about 47.degree. F.). Thus, the hazards related to fire and explosion are not relieved by the inclusion of the above noted reactive diluent. Moreover, when curing the compositions of the Broderick et al. patent at elevated temperatures, a considerable amount of the methyltrimethoxysilane has been found to volatilize, even though it is intended to react with the other components. This evaporation of reactive diluent not only wastes a costly component, but contributes to total volatile organic compounds (VOC) emission, counter to current trends in air quality standards.